ESR dating of corals and pre-annealing effects on ESR signals

Quaternary Science Reviews, Vol. 11, pp. 203-207, 1992. Printed in Great Britain. All rights reserved.

0277-3791/92 $15.D0 (~) 1991 Pergamon Press pie

ESR DATING OF CORALS AND PRE-ANNEALING EFFECTS ON ESR SIGNALS S. Ikeda, M. Kasuya and M. Ikeya Department of Physics, Faculty of Science, Osaka University, Machikaneyama, Toyonaka, Osaka, 560Japan

Corals c o m m o n l y exhibit electron spin resonance (ESR) signals of A (g = 2.0057), B (g = 2.0031), and C (g = 2.0007). Preannealing using a m o d e r n coral sample at 120-180°C has significantly changed g a m m a - r a y sensitivities of the E S R signals. The sensitivities of all the three signals at first increased and then decreased with the pre-annealing. These results suggest that the a m o u n t of sources of the signals changes with geologic time. This effect can cause underestimation of E S R ages for Holocene and latest Pleistocene corals, and can cause overestimation for Early and Middle Pleistocene corals using signal C. Possible models associated with a m o n o m e r and a trimer of cation impurities for signals C and A, respectively, are discussed based on third order trap decay kinetics.

INTRODUCTION Age determination of aragonitic corals is very important to Quaternary geochronology because the formation of coral reefs is closely related to climatic and tectonic history. Corals are revealed to be useful for electron spin resonance (ESR) as well as U-series dating (Ikeya and Ohmura, 1983; Koba et al., 1985; Skinner, 1985; Radtke and Griin, 1988). Corals commonly exhibit ESR signals of A (g = 2.0057), B (g = 2.0031), and C (g = 2.0007) (Fig. 1; Ikeya and Ohmura, 1983). Other carbonate materials (molluscs, foraminifera, and calcite speleothems) often exhibit similar signals at g = 2.0057 (hi) and g = 2.0007 (h3) (Yokoyama et al., 1983; Skinner, 1983; Katzenberger et al., 1989). ESR intensities of the three signals A, B, and C increase with time. Signals B and C are enhanced by artificial gamma-rays, but the intensity of signal A does not increase with simple gamma-ray irradiation. Signal C has been practically used for dating because it grows

A i

2.015[ Z010I Z0051

1 mT

i

2.0001 t9pi5 g-value

FIG. 1. Typical E S R spectrum of a fossil aragonitic coral. The gvalues of signals A, B, and C are g = 2.0057, 2.0031, and 2.0007, respectively.

with applied radiation. On calcite speleothems, Yokoyama et al. (1983) suggested that signal A should be used for dating after a thermal annealing. They considered that the thermal annealing at 170-185°C for 16 hr transferred electrons of signal C (h3) to the signal A (hi) site. The origins of these signals have been controversial. Signal A was attributed to humic acid which is commonly included in natural carbonate (Griin and De Canniere, 1984). The idea, however, has been criticized because the signal is also observed in synthetic calcium carbonate (De Canniere et al., 1985; Barabas et al., 1989). Signal B was attributed to organic radicals (Ikeya, 1981). Signal C was considered to be due to CO33- radicals (Ikeya and Ohmura, 1983). Barabas et al. (1989), on the other hand, stated that the nature of this signal was probably associated with Mg 2+ Yokoyama et al. (1988) and Rossi and Poupeau (1989) reported using calcite single crystals that both signals A and C have isotropic g-factors. Yokoyama et al. (1988) considered that signals A and C were due to the electrons trapped in the amorphous region which was formed by fission-fragments or alpha-recoil particles as suggested by Ikeya (1978). Recently, signal C is considered to be due to rapidly rotating CO2- stabilized by some impurity, from the observation of hyperfine structures of 13C (Debuyst et al., 1991) and also from the temperature dependence of the signal (Miki and Kai, 1990). Wieser et al. (1985) reported that signal A in calcite speleothems increased with heating. Katzenberger et al. (1989) also reported that signal A in Recent molluscs grew with annealing. But no detailed study on pre-annealing effects on ESR signals of corals has been reported. The present study observed pre-annealing effects on the sensitivities of gamma-ray induced ESR signals of corals to investigate the nature of the paramagnetic centers causing the signals and to investigate if the effect has any influence on the practical ESR dating.

203

204

s. lkeda et al.

EXPERIMENTAL A modern coral sample (Acropora sp.) was powdered and sieved between 0.25-0.50 mm in diameter. Organic matter attached to the sample was dissolved in oxygenated water. ESR spectra were measured at room temperature with an X-band spectrometer (JOEL-REIX) with a modulation frequency of 100 kHz and a modulation width of 0.1 mT. The modulation width is larger than the linewidth of the signal B but still within the range where the peak to peak derivative amplitude is indicative of the signal intensity. A microwave power of 1 mW was used for the measurement. The relationship between gamma-ray dose and ESR signal intensity was examined using the sample which was pre-heated. Three batches from the sample were pre-annealed for 1 hr at 120, 160,200°C, and one batch of unannealed control was also prepared. Each batch was split into 6 aliquots, and the aliquots were irradiated with different gamma-ray doses of 0-500 Gy from the source of ~'°Co. Pre-annealing and subsequent artificial gamma-ray irradiation of 1 kGy were carried out on the same sample; the dose is high enough as to create the saturation values of the ESR intensities of corals. The aliquots were pre-heated at 12(I°C (0-64 hr), 140°C (0-16 hr), 160°C (0-4 hr), and 180°C (0-4 hr) before irradiation. The standard error of a signal intensity was examined by repeatedly measuring 20 aliquots of the same batch irradiated with a gamma-ray dose of 500 Gy. The resulting standard error (lo) was about 1.5%. X-ray diffraction measurements were carried out to check that there was no recrystallization for the annealed aliquots. EXPERIMENTAL RESULTS The change of the ESR signal response by irradiation due to thermal pre-heating of the coral was studied. (a) Signal Growth on Irradiation The growth of the ESR intensities of signals A, B, and C with artificial gamma-ray dose for the unannealed batch of the sample and for the batches which were pre-heated at 120, 160, and 200°C is shown in Fig. 2 (a, b, and c). Signal A of unannealed aliquots was not enhanced by artificial gamma-ray irradiation. On the other hand, the signal of pre-annealed aliquots was increased by gamma-ray irradiation. The gamma-ray sensitivity for signal B of annealed aliquots was increased at 120°C (about twice as large as that of unannealed aliquots) and decreased at 160°C. The sensitivity of signal C at 120°C was slightly larger than that of unannealed aliquots, and it decreased at 160°C. (b) Dependence of Pre-annealing Time ESR intensities of signals A, B, and C for the modern coral which was pre-annealed at 120, 140, 160, and 180°C and irradiated subsequently with a gamma-

I

,

I

4 3

//

2OO'C

2'

120"C

i/mj ~mr~

0 ~ ,

una~aled

~

,

(b) ¢o

ffl C

r-----

/z-~fe~

-~

4O /i 2O

r-

o-10 w 0

I

I

*

I

I

I

f

I

(c) 80 60 40 20

0

100 200 300 400 500 Gamma-ray Dose.(Oy)

FIG. 2. Growth of the ESR signal intensities by artificial gammaray dose for unannealed aliquots and aliquots which were preannealed at 120, 160, and 2(X)°C. (a) Signal A (g = 2.0057) of unannealed aliquots was not enhanced by artificial gamma-ray. O n the other hand, the signal of pre-annealed aliquots was increased by gamma-ray. (b) The intensities of signal B (g = 2.0031) of annealed aliquots were increased at 120°C (about twice as large as that of unannealed aliquots) and were decreased at 160°C. (c) ESR intensity of signal C (g - 2.0007) was increased at 120°C and decreased at 160°C.

ray dose of 1 kGy are shown as a function of the annealing time in Fig. 3 (a, b, and c); the radiation dose is high enough as to cause the saturation of growth in the signal intensity versus dose curve. Hence, the intensities shown in the figure are related to the amounts of traps of the signals A, B, and C. ESR intensity of each signal was changed by the preannealing prior to the irradiation. The intensities of all three signals at first increased and then decreased as the pre-annealing time increased. The rate of" initial enhancement in the ESR intensity increased with increased pre-annealing temperature. Signal A, whose intensity usually decreases with artificial gamma-ray, was produced by the annealing without any artificial irradiation. Artificial gammairradiations increased the signal intensity furthermore. The intensity was decreased by prolonged preannealing at 160 and 180°C. The behaviors of signals B and C resemble each other. The intensities of signals B and C were increased by pre-annealing at 120°C. The intensities which pre-annealed at 140, 160, and 180°C were once increased but decreased with longer annealing control.

205

ESR Dating of Corals

~

2

r6o*c

1

~4o'c

(a)

~2o'c

4|

0

o ~c

E" tv" t/3 I.iJ i

0

i

5

rl

I0

I'5

Time (hour) FIG. 3. Change in ESR intensities of (a) signal A, (b) signal B, and (c) signal C by heat treatment before irradiation. A coral sample (Acropora sp.) was pre-annealed at 120; 140, 160, and 180°C and subsequently irradiated with an artificial gamma-ray dose of 1 kGy, which saturated the signal intensities. The estimated standard error of an ESR intensity is about 1.5%. Error bars are smaller than the size of dots in this figure.

the traps in Fig. 3 can be explained well if we assume third order decay. Figure 4 (a and b) show first, second, third, and fourth order decay curves using appropriate values of K1 and K2. The third order decay curve fits the experimental result better than the other orders. We assume that the sources for signals B and C decay in third order. It is well known in solid state physics that some impurity centers decay in third order and aggregate to form stable trimers (Ikeya and Crawford, 1973). The rigid solution of the rate equations above is difficult. Hence we used an approximation to Eq. (2) by changing Kt, K2 and N l to find appropriate values. An Arrhenius plot of the rate constants Kt and K2 of each annealing temperature gave the extrapolated constants at 20°C as K~ = 3.7 x 10 -s sec -1, K2 = 1.6 × 10-18 sec -t for signal B and K1 = 1.1 × 10 -9 sec -l, K2 = 4 . 0 X 10 -16 s e c - 1 for signal C (Fig. 5). Using the estimated K1 and/(2 for signal C at 20°C and the above equations, we can deduce the variation in number of traps (N2) as a function of geologic time for the last million years (Fig. 6). Though K~ and K2 at 20°C contain large errors, overall tendency would be close to the curve. The number of traps for signal C drastically decreases in the first 100 thousand years. The number of traps in a sample at a certain geologic age is in proportion to the ESR sensitivity for an artificial gamma-ray when we estimate the total dose (TD) of natural radiation. Hence, the variation can cause underestimation in ESR ages using signal C for Holocene and Latest Pleistocene corals and can cause overestimation for Early and Middle Pleistocene cor-

(a) signal B

DISCUSSION The gamma-ray sensitivities of the ESR signals A, B, and C at first increased and then decreased with the pre-annealing time. This indicates that the number of traps causing each signal is not constant but it varies as a function of time and temperature. That the rates of the increase and decrease are high at higher temperatures suggests the presence of some thermally activated process for the formation and destruction of traps. Formation and decay of the traps can be described by considering the interaction of traps. The numbers of traps for signals B and C may be explained by N2 of the following rate equations: dNt/dt = - K 1 N I dN2/dt = KiN1

-

K2(N2) i

dN3/dt = K2(N2)'

10

~5, I

I

'b) signal C

¢-

n~ If) lJJ

(1) (2) (3)

where Ni is the number of the main source from which N2 is formed, N2 is the number of the paramagnetic centers, and N3 is the number of traps formed by the decay of N2. K1 and K2 are rate constants of the two reactions and i is the order of the reaction. Experimental results of the formation and decay of

I

0

5

I

lC)

Time (hour)

15

FIG. 4. First, second, third and fourth order decay curves fitting for gamma-ray induced ESR intensities pre-annealed at 140°C: (a) signal B and (b) signal C. The third order decay can better explain the experimental results.

206

S. Ikcda et al.

180160140120 0

,

i

;

20(°C)

;

i

KI

a

--75

7

,--c-O--e-, x,

•

/

-10

L l

\a, \

\

\,

--\

® cO@ c; +Cal I0[4 co - cg',! 1;I}UF]I.~

-20

'

2.5

Z0

lO00/r

3.0

3.5

FIG. 5. Arrhenius plot of the rate constants Kt and K~ ol each prcannealing temperature. Extrapolation to 20°C gives the approximate values of K~ and K~.

ro

b

~'6N201

@oQ@

b .CI

E Z

[i

2+

Ca cation CO -

impurity

i '

ii L.

o

266---

So

,--

~

86o

J

ooo

CO -

FIG. ?. Possible models for ESR signals ol aragonitic corals. (a) A cation impurity near by a rotating C O , can cause signal C. (b) A trilncr of cation imp~',rilies in the ((XI1) plane may cause signal A.

-rime (lO~r) FIG. 6. Variation in n u m b e r of traps at 2(I°C as a function of gcolo -:: timc using K~ and K, of signal ('. N2, m e a n s the initial value of/'d~ The n u m b e r of traps for signal C decreases drastically in the first l'n0 thousand years. Some correction is necessary around this regio,L

als. Present argument stands on the assumption th~,.t filled traps are stable and the number of empty traps decays in third order to form aggregation of impurities. We propose possible models of the ESR signals of corals. The change in the formation efficiency of the paramagnetic centers seems to be related to the decomposition of organic materials, decomposition whtch supplies sites or stabilizers for the signals A, B, and C. The numbers of traps for signals B and C at first increase and then decay in third order kinetics, which indicates the sources of signals B and C decay due to the trimer formation of impurities. Cation impurities which can form a trimer in the (00l) plane in the aragonite lattice, are expected, as shown in Fig. 7 (a and b). Signal C is considered to be due to rapidly rotating CO2- stabilized by some impurity (Debuyst et al., 1991; Miki and Kai, 1990). The source of signal A may be related to the trimer of such impurities associated with signal C. The increase in the source for

signal A corresponds to tile decrease for signal C. Hence, we might suggest that the trimer is related to the source of the traps for signal A: the transfer of signal C to A has been proposed by Yokoyama et al. (1983). The present study shows the importance of the change in ESR sensitivity of corals in geologic time. The change is presumably related to the decomposition of organic materials and to impurity aggregation into trimers in the skeleton of corals. The third order decay model for the traps associated with signals B and C is a tentative model to explain the experimental results and to assess the errors involved in ESR ages. A model of a trimer formation is also a speculative one based on the above tentative model. Therefore, the present work is a working hypothesis for the refinement of ESR ages and for the possible assessment of the paleoclimate. ACKNOWLEDGEMENTS We thank Mr S. Toyoda of Osaka University for helping us with the error calculation. G a m m a - r a y irradiations and X-ray measurements were carried out at The Research Laboratory of Science and Industry, and The D e p a r t m e n t of Chemistry, Osaka University, respectively.

ESR Dating of Corals

REFERENCES Barabas, M., Bach, A., Mudeisee, M. and Mangini, A. (1989). Influence of the Mg-content on ESR-signals in synthetic calcium carbonate. Applied Radiation and Isotopes, 40,. 1105-1111. Debuyst, R., Bidiamambu, M. and Dejehet, F. (1991). An EPR study of ?- and a-irradiated synthetic powdered calcite labelled with 13C. Nuclear Tracks and Radiation Measurements, 18, 193201. De Canniere, P., .loppart, T., Debuyst, R., Dejehet, F. and Apers, D. (1985)." ESR dating: A study of humic acids incorporated synthetic calcite. Nuclear Tracks and Radiation Measurements, 10, 853-863. Griin, R. and De Canniere, P. (1984). ESR-dating: problems encountered in the evaluation of the naturally accumulated dose (AD) of secondary carbonates. Journal of Radioanalytical Nuclear Chemistry Letters, 85, 213-226. Ikeya, M. (1978). Electron spin resonance as a method of dating. Archaeometry, 20, 147-158. Ikeya, M. (1981). Paramagnetic alanine molecular radicals in fossil shells and bones. Naturwissenshaften, 67, 474. Ikeya, M. and Crawford, ,I.H. (1973). EPR study of impurityvacancy aggregates in NaCi containing both Mn 2÷ and other divalent cations. Physical State Solids (b), 58, 643q554. lkeya, M. and Ohmura, K. (1983). Comparison of ESR ages of corals from marine terraces with 14C and 23°Wh/234U ages. Earth and Planetary Science Letters, 65, 34-38. Katzenberger, O., Debuyst, R., De Canniere, P., Dejehet, F., Apers, D. and Barabas, M. (1989). Temperature experiments on

207

mollusc samples: an approach to ESR signal identification. Applied Radiation and Isotopes, 40, 1113-1118. Koba, M., Ikeya, M., Miki,T. and Nakata, T. (1985). ESR ages of the Pleistocene coral reef limestones in the Ryukyu Islands, Japan. In: Ikeya, M. and Miki, T. (eds), ESR Dating and Dosimetry, pp. 93-104. Ionics, Tokyo. Miki, T. and Kai, A. (1990). Rotating CO 2 centers in coral and. related materials. Japanese Journal of Applied Physics, 29, 21912192. Radtke, U. and Griin, R. (1988). ESR dating of corals. Quaternary Science Reviews, 7, 465-470. Rossi, A.M. and Poupeau, G. (1989). Radiation-induced paramagnetic species in natural calcite speleothems. Applied Radiation and Isotopes, 40, 1133-1137. Skinner, A.F. (1983). Overestimate of stalagmitic calcite ESR dates due to laboratory heating. Nature, 304, 152-154. Skinner, A.F. (1985). Comparison of ESR and 23°Th/234U ages in fossil aragonitic corals. In: Ikeya, M. and Miki, T. (eds), ESR Dating and Dosimetry, pp. 135-138. Ionics, Tokyo. Wieser, A., GOksu, H.Y. and Regulla, D.F. (1985). Characteristics of gamma-induced ESR spectra in various calcites. Nuclear Tracks and Radiation Measurements, 10, 831-836. Yokoyama, Y., Quaegebeur, ,I.P., Bibron, R. and Leger, C. (1983). ESR dating of paleolithic calcite: thermal annealing experiment and trapped electron lifetime. PACT, 9, 371-379. Yokoyama, Y., Bibron, R. and Leger, C. (1988). ESR dating of palaeolithic calcite: a comparison between powder and monocrystal spectra with thermal annealing. Quaternary Science Reviews, 7, 433-438.